Naval vessels use flexible towed arrays of hydrophones for determining the presence and location of targets emitting or reflecting acoustical energy in the vicinity of the towing vessel. Typically a towed array is wound on a spool and stored on the ship for deployment at sea when needed for target detection and location. These towed arrays are generally of flexible tubing and contain discrete acoustical sensors distributed along the array which can be a mile or more in length.
A prior design for towed arrays would have acoustical sensors positioned along the length of the array which are linked for communication to a computer on the towing vessel. The computer processes the multiplicity of information obtained from the sensors along the full length of the array. When an acoustic signal emitted or reflected off a target is detected by the acoustical sensors, this information is used in the onboard computer to obtain an estimate of the detected target position.
More recently target detection and location has been improved to include towed array heading information obtained from outputs of magnetic sensors positioned at discrete points along the array. Discrete towed array heading information enables improved position estimates for the acoustical sensors which, in turn, provides phase corrections for the sonar data and allows improved system performance.
Magnetic heading sensors which have been used in towed arrays can determine magnetic heading but are limited in accuracy and bandwidth. The accuracy of the magnetic heading measurement is strongly influenced by known and unknown local magnetic anomalies. Typically the geometry of the array is varying dynamically, predominantly along a direction lateral to the course of the towing vessel. This effect is greatest during turns. Limited stretching also occurs down the towed array. Further, significant lateral motions occur due to cross currents. At polar latitudes, random disturbances of the magnetic field increase due to environmental influences with an accompanying degradation in accuracy of the magnetic heading sensors. Finally, the magnetic heading sensor measurements take from seconds to minutes to stabilize after dynamic changes caused by the factors discussed above.
This time lag for determining the lateral position of the towed array is too long for many applications. Therefore, magnetic sensors perform inadequately in most dynamic situations.
To achieve accurate targeting, it is necessary to have accurate estimates of the actual location of each of the acoustical sensors positioned at intervals along the towed array.
The use of inertial heading sensors as an improvement over magnetic heading sensors has been suggested. However, not until recently has inertial technology progressed to a point where the size and quality of inertial sensors make such usage feasible. Even with the modern inertial sensor technology there remained, until now, the unsolved problem of providing initialization, or alignment, of an inertial sensor in the undulating, rotating towed array at a remote point from the source of reference.
Heretofore, other techniques have been used to obtain alignment of remote or slave inertial systems on naval vessels. The problem has been to obtain an estimate of the heading of a remote object (slave) with respect to the ship's master or reference inertial system. For example, between a ship (master) and a missile (slave) mounted upon the ship's deck.
Generally, since both the master unit and slave unit have inertial sensors for determining orientation, known relative position (lever arm) and orientation between the master and slave unit can be used in establishing the initial position and alignment of the slave inertial system. The initial alignment of the slave inertial system using this method is generally not accurate due to so called boresight error and lever arm flexure.
Subsequently, dynamic changes in the differences between the error in computed position of the master and slave inertial systems can be observed via comparison. These differences may be processed through a Kalman "transfer alignment" filter. From this processing, corrections can be made to modeled error states, such as, slave position, velocity, heading and attitude. By this method, the inaccurate initial alignment described above is much improved. Such position comparison systems were hereto-fore directed to those situations where the locations of the master and slave units relative to one another, did not vary with time.
Determining the position, velocity, heading and attitude of points along a towed array is a more difficult problem as the relative displacements between the master inertial system on the vessel and the slave inertial systems in the towed array vary with time. Thus the standard transfer alignment mechanization may not be used for a towed array application.